Bischofite

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Bischofite[1][2]
Mineralogical name Bischofite
Chemical name Magnesium chloride hexahydrate
Trivial name
Chemical formula MgCl2•6H2O
Other forms
Crystal system monoclinic
Crystal structure
Deliquescence humidity 20°C 33.1%
Solubility (g/l) at 20°C 5.75 mol/kg
Density (g/cm³) 1.57 g/cm3
Molar volume 129.6 cm3/mol
Molar weight 203.30 g/mol
Transparency transparent to translucent
Cleavage none
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments decomposes at 116-118°C
hygroscopic and deliquescent
Crystal Optics
Refractive Indices nx =1.495
ny = 1.509
nz = 1.528
Birefringence Δ = 0.003
Optical Orientation positive
Pleochroism
Dispersion 79°
Used Literature
[Broul.etal:1981]Title: Solubility in organic two component systems
Author: Broul M., Nyvlt J.; Soehnel O.
Link to Google Scholar
[Dana:1951]Title: Dana's System of Mineralogy
Author: Dana J.D.
Link to Google Scholar
[Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
Link to Google Scholar



Solubility

Bischofite has got a high solubility in water (5.75 mol/kg at 20 °C). The solubility of bischofite and other phases of the system MgCl2-H2O is shown in Fig. 1.

Figure 1: Solubility of magnesium chloride in water. The molality m [n(MgCl2)•kg(H2O)-1] is plotted versus the temperature.


Hygroscopicity

Figure 2 includes the phase diagram of the system MgCl2-H2O. Next to the hexahydrate bischofite there are two more stable phases within the regarded temperature range. The octa- and the dodecahydrate are relevant at lower temperatures.

Figure 2: Phase diagram of the system MgCl2-H2O.The water activity aw is plotted versus the temperature.



Table 1: Deliquescence humidities of bischofite at different temperatures [Steiger.etal:2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
Link to Google Scholar
0°C 10°C 20°C 30°C 40°C 50°C
34.1%r.h. 33.7%r.h. 33.1%r.h. 32.4%r.h. 31.5%r.h. 30.5%r.h.


Weblinks

Literature

[Broul.etal:1981]Elsevier (eds.) Broul M., Nyvlt J.; Soehnel O. (1981): Solubility in organic two component systems, ElsevierLink to Google Scholar
[Dana:1951]Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & SonsLink to Google Scholar
[Steiger.etal:2011a]Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy (2011): Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars. Geochimica et Cosmochimica Act, 75 (12), 3600-3626, 10.1016/j.gca.2011.03.038,Link to Google Scholar
[Steiger.etal:2014]Steiger, Michael; Charola A. Elena; Sterflinger, Katja (2014): Weathering and Deterioration. In: Siegesmund S.; Snethlage R. (eds.): Stone in Architecture, Springer Verlag Berlin Heidelberg, 223-316, 10.1007/978-3-642-45155-3_4Link to Google Scholar